The present invention relates to improvements in a squelch circuit used for the reception of angle-modulated signals.
In an angle-modulation communication system such as an FM (frequency-modulation) or a PM (phase-modulation) communication system, there is a noise suppression effect depending on the modulation index. Therefore, a fairly high signal-to-noise ratio (S/N) can be achieved for the demodulated signal. However, if the carrier power to noise power ratio (C/N) becomes small, the S/N of the demodulator output is lowered abruptly, making the normal reception of signals impossible.
Heretofore, for the purpose of resolving this problem, a squelch circuit has been employed, which comprises means responsive to the noise level for interrupting the demodulated signal to mitigate noise interference when the noise level is high.
This squelch circuit is widely used in amature radio communications or the like, and is effective for one-way communication systems. However, the precision in the level detection is low and the response time is long. For instance, in a burst signal transmission system in which a carrier wave is transmitted only during the period when a voice waveform is present, the presence or absence of a carrier wave is determined depending upon the output noise power at the receiver circuit. Accordingly, there is a disadvantage that the precision in detection is not sufficiently high, and that the speed is not high enough to follow the voice waveform.
In addition, another squelch circuit is known, in which a carrier energy band compression effect of a phase synchronizing circuit is utilized, and by effecting homodyne detection, carrier power is detected to intercept a demodulated signal and to mitigate noise interference (reference is made to U.S. Pat. No. 3,397,360). In this case, there is a disadvantage that although the carrier can be detected if the C/N is sufficiently large (experimentally, if it is about 3 dB or larger), malfunction may arise in the detection of the carrier power if the C/N becomes small.